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Topic Review (Newest First)

11-02-2010 07:01 PM

SoulVoyage

Anybody have any experience using the Pur 35watermaker? I would be interested in this one, because it can also be used in manual-mode, albeit FAR slower and lotsa work. I think the sell for $1800, but much less used...just hard-to-find used.

11-01-2010 09:44 PM

GaryHLucas

By the way, run that motor on 230 Volts if your generator produces it! Otherwise you'll drawing twice as much current on half of the generator windings and that will surely cause you trouble.

I once went on a trouble call (industrial electrician in a former career, this is #3) on the biggest sump pump I've ever seen in a basement, a 4" discharge! It was overloading the lady's backup generator every time it kicked on. IT was 120 volts, but the generator was 120/240. I installed a 240 to 120 volt transformer and used 240 from the generator and the problem was solved.

Gary H. Lucas

11-01-2010 09:36 PM

GaryHLucas

The 800 psi is not a test pressure. We are REVERSING osmosis, the natural tendency for a liquid with a low concentration to permeate through a membrane into a higher concentration thereby diluting it. We need to force the liquid in a high concentration (seawater salt) to pass into a low concentration ( the clean permeate) Osmosis is so powerful it pushes water from the roots to the tops of giant redwoods!

So you need the 800 psi to overcome the natural osmotic pressure of seawater. If the pressure is lower permeation will happen but more salt will be passed through too. RO elements are never 100% filters. They may be 99.8% for seawater at the right pressure. On glycol they pass 7%, and we use three stages to get 7% of 7% of 7% for the final discharge.

The rate of permeate discharge is not related directly to the feed flow in GPM. Slowing down the flow using a smaller pump at the same pressure will produce the same permeate rate, for a while. The concentration of salt left behind will be higher, more likely to precipitate instead of staying in solution, and there will be reduced flushing, all leading to fouling quickly.

Baldor premium efficiency motors are very good, and increasing efficiency to get a lower amp rating is a good idea and not too expensive at this size motor. I don't work with single phase motors much any more. If this were a 3 phase motor I'd put a soft starter on it. That would allow you to use almost all the horsepower of the generator because it would be easy to start he motor. You might look at using a 3 phase motor (cheaper, lighter and more robust) with an inverter GS2-23P0 $300 from AutomationDirect to run it from the generator. You could then ramp up the motor gently and exactly match the required pump speed too. They have great tech support, you could discuss this with them.

Gary H. Lucas

11-01-2010 03:04 PM

Cruisingdad

Another question on watermakers:

If I reduce the GPM, that will only reduce the output, right? If I want max output, I must maintain max GPM? But if my putput drops to say, 2.8 gpm versus 3-4, but I maintain my 800+ psi, I will only make slightly less than rated water, correct?

Brian

11-01-2010 02:54 PM

Cruisingdad

Open question:

I have a bit of a problem to work through.

I have a 3.5kw generator. That would be my preferred method of running the watermaker. However, I will be limited to 26 amps max. In reality, I should keep it under 23 or lower, with lower being mych preferred. I have the HP on my main engine to run a watermaker, but unfortunately, not much room. So the preference really would be a Generator run watermaker.

So, most of the watermakers call (ideally) for 3-4 gpm with 1000 psi. That will be difficult to obtain and stay under my parameters as I suspect that will take a 2.5 hp motor. As such, do you (G LUcas or Rich H or others) feel that I can run a 2.5 X 40 with 3-4 gpm and 800 psi? I hav been told, not sure it was reliable, that 800 psi was the test pressure for most membranes and should be fine.

Also, regarding motors, I have several to choose from. Rich suggested a Giant motor which only pulls 17.5ish amps. Another option might be this Baldor EL1405T:

I am researching building my own watermaker. If I do it, I will detail the process here and what I bought and how I did it.

One question: On the WM's you have helped put together, did you use a electric motor or engine driven?

Also, who would you reccomend for the membrane? I have a couple of sites I am looking at, but curious who has bought/has experience with them?

Thanks.

Brian

10-08-2009 09:28 PM

GaryHLucas

I understood your layout, I was just suggesting that the manifold could be an open vented tank so you don't need the relief valve. The flow rate is so low it would never fill the tank unless your lines were really tiny.

Yes, I would put two pressure gauges, one on the feed end one on the concentrate end. You want some pressure drop across the membrane. If there isn't any pressure drop then there isn't any turbulence and it will foul rapidly. There is a minimum GPM through flow for each size and type of membrane. So the two gauges tell you that you have sufficient flow, and then that the membrane is fouling.

I'd try to do it without the relief valve, by running the permeate into a vented tank or open top PVC standpipe, then gravity flow through 4 inexpensive plastic valves that will see no pressure. You want to maximize your clean water production and backpressure on the permeate lines always reduces it.

Gary, seems my "layout" wasn't that clear. The permeate would flow into the manifold and four valves (taps) which would allow me to direct the flow (not under pressure) to where I want it. The relief valve is simply to ensure that I don't accidentaly pressurise the inside of the mambrane by shutting off all 4 valves.

One last thing. When the differential pressure from the feed end to the discharge end starts to increase you have fouling going on. The membrane should be cleaned immediately, or you run the risk of never being able to clean it. If you clean it and the differential pressure is still high you didn't get it clean, and another immediate cleaning is warranted. Often you need to change cleaning chemistries, because the foulant may be something you weren't expecting. Membranes have a bad tendency to get into a death spiral. One bad cleaning leads to another until the membrane is shot.

The system I am contemplating has only one presssure guage - on the brine line at the output end of the membrane (to adjust the pressure on the high pressure side of the membrane). How do you know that a differential pressure exists? That implies two pressure guages. Please explain the setup as you see it.

Thanks

10-07-2009 09:11 PM

GaryHLucas

I'd try to do it without the relief valve, by running the permeate into a vented tank or open top PVC standpipe, then gravity flow through 4 inexpensive plastic valves that will see no pressure. You want to maximize your clean water production and backpressure on the permeate lines always reduces it.

Another interesting thing. Membranes will produce poor quality water if run at too low a pressure. You can get water out of a membrane at almost any pressure, but the water will be the highest quality when the pressure matches the membranes design pressure.

If you take your boat up a river into fresh water you'll find that the RO will produce a lot more permeate at a much lower pressure, like 300 psi. You wouldn't want to run it at 800 psi because the flux rate (permeate flow) would be way to high and you'd run the risk of fouling the membrane.

One last thing. When the differential pressure from the feed end to the discharge end starts to increase you have fouling going on. The membrane should be cleaned immediately, or you run the risk of never being able to clean it. If you clean it and the differential pressure is still high you didn't get it clean, and another immediate cleaning is warranted. Often you need to change cleaning chemistries, because the foulant may be something you weren't expecting. Membranes have a bad tendency to get into a death spiral. One bad cleaning leads to another until the membrane is shot. On a boat you are talking $100 to maybe $700. On our systems it is more like $10,000!

Gary H. Lucas

10-07-2009 12:01 AM

Omatako

Gary

I intend to have my brine line going out above the water line to prevent it cloggng with sea life etc and to save having another below waterline skin fitting.

On the permeate line I intend to build a stainless manifold. I have three tanks so I want to have the manifold equipped with obviously an incoming line from the membrane cylinder and then four valved outlets. One will go to a sampling tap to be able to ensure that the water is good before sending it to a tank. The other three will go to each of the tanks so that I can fill the one that needs it.

On this manifold I will also put a relief valve that will vent at about 200PSI to prevent any pressure build up if something goes wrong downstream or if all 4 valves are accidentally turned off.

Is this OK or am I over-killing the thing?

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